1. Field of the Invention
The present invention relates to diamond and its preparation by a chemical vapor deposition method using a low temperature plasma. More particularly, the present invention relates to a method for producing the high quality diamond or covering a substrate having the high quality diamond at a high rate with a low temperature plasma. The present invention is applied in various fields which require high heat conductivity, low dielectricity, good optical transmission, high specific elasticity,.high strength, good wear resistance, etc, such as window materials, diaphragms, cutting tools, heat sinks, IC bonders and the like.
2. Description of the Related Art
Hitherto, the diamond is synthesized in a thermodynamically equilibrium state at high temperature under high pressure. Recently, the diamond can be synthesized by a chemical vapor deposition (CVD) method which positively utilizes an non-equilibrium state.
As the CVD methods for synthesizing the diamond, various methods comprising exciting a mixture of a hydrocarbon and a ten time volume of hydrogen with plasma or a heating filament. For example, Japanese Patent Kokai Publication No. 91100/1983 discloses a method comprising preheating the mixture of the hydrocarbon and hydrogen with a thermionic emission material kept at a temperature not lower than 1,000.degree. C. and introducing the preheated mixture onto a heated substrate surface to deposit the diamond through pyrolysis of the hydrocarbon. Japanese Patent Kokai Publication No. 110494/1983 discloses a method comprising flowing the hydrogen gas through microwave electrodeless discharge, mixing the hydrogen gas with the hydrocarbon and then depositing the diamond on the substrate through pyrolysis of the hydrocarbon. Japanese Patent Kokai Publication No. 3098/1984 discloses a method comprising introducing a microwave in the mixture of the hydrocarbon and hydrogen to generate a plasma, heating a substrate placed in the plasma at a temperature of 300.degree. to 1,300.degree. C. to deposit the diamond on the substrate through pyrolysis of the hydrocarbon.
Since the conventional CVD methods for synthesizing the diamond use the mixture essentially consisting of hydrogen and a carbon-containing compound (for example, hydrocarbons), the plasma is stably generated at a comparatively low pressure up to about 50 Torr. Therefore, synthesis conditions, synthesis rates and synthesis areas are restricted, which limits application of the diamond synthesis by the CVD methods.